The present invention relates to an imaging lens for forming an image of an object on an imaging element such as a CCD sensor and a CMOS sensor, and particularly, it relates to an imaging lens suitable for mounting in a relatively small camera such as a cellular phone, a digital still camera, a portable information terminal, a security camera, a vehicle onboard camera, and a network camera, and an imaging device equipped with the imaging lens.
In these years, in place of cellular phones that are intended mainly for making phone calls, so-called “smart phones” have been more widely used, i.e., cellular phones with such functions as those of portable information terminals (PDA) and/or personal computers. Since the smart phones generally are highly functional as opposed to the cellular phones, it is possible to use images taken by a camera thereof in various applications. For example, in case of the smart phone, while it is possible to use for printing and enjoying images taken as of its intended use, it is also possible to use in additional uses such as processing images to be used in games or for makeup simulations, dress fitting simulations, and the others. Such uses of the images, which were not conventionally common, are becoming increasingly popular every year.
Generally speaking, an imaging lens to be mounted in the cellular phone or the smart phone, which is developed for advanced users, is required to have a high resolution lens configuration so as to be also applicable to a high pixel count imaging element of these days. However, as the imaging lens to be mounted in a camera used for the above-described usages, it is critical to be a small size with a wide angle of view, that is, a wide angle, than having a high resolution.
Conventionally, there have been proposed various lens configurations, and among them, an imaging lens having a three lens configuration has been used in a number of cameras because of its relatively satisfactory ability of correcting aberrations and suitability for downsizing. As the three-lens configuration, there has been a well-known lens configuration, which includes a first lens having positive refractive power; a second lens having negative refractive power; and a third lens having positive or negative refractive power. In the lens configuration, since the second lens has negative refractive power, a chromatic aberration occurred in the first lens is suitably corrected through the second lens and it is suitable for downsizing of the imaging lens.
However, in the lens configuration, since the second lens has negative refractive power, when it is attempted to attain both downsizing of the imaging lens and a wider angle, refractive power of each lens tends to be relatively strong. As a result, it is necessary to improve the fabrication accuracy and assembly accuracy of the lenses. For this reason, there has been recently proposed a lens configuration, in which refractive powers of all of the three lenses are positive.
As the imaging lens having the three-lens configuration, for example, an imaging lens described in Patent Reference has been known. According to the imaging lens, downsizing of the imaging lens and a wider angle are attained, while correcting aberrations by providing an inflexion point in the second lens and the third lens.    Patent Reference: Japanese Patent Application Publication No. 2012-14139
According to the imaging lens described in Patent Reference, it is possible to attain downsizing of the imaging lens while correcting aberrations to some extent. In the imaging lens, however, since the whole lens system has a long focal length, a length on an optical axis tends to be long and it is difficult to fully meet the recent demands for downsizing. In addition, with diversification of camera uses, imaging lenses are demanded to have even wider angles. Although the imaging lens described in Patent Reference has the wider angle of view than that of conventional one, there is a limit by itself for attaining even a wider angle.
Here, a difficulty of attaining both downsizing and the wider angel while satisfactory correcting aberrations is not a problem specific to the imaging lens to be mounted in cellular phones and smart phones. Rather, it is a common problem even for an imaging lens to be mounted in a relatively small camera such as digital still cameras, portable information terminals, security cameras, onboard cameras, and network cameras.
In view of the above-described problems in conventional techniques, an object of the present invention is to provide an imaging lens that can attain downsizing and a wider angle while satisfactorily correcting aberrations, and an imaging device equipped with the imaging lens.
Further objects and advantages of the invention will be apparent from the following description of the invention.